CT, MRI, and 18F-FDG PET/CT imaging features of seven cases of adult pancreatoblastoma

Objective This study mainly analysed the imaging data for seven cases of adult pancreatoblastoma (PB) and summarized additional imaging features of this disease based on a literature review, aiming to improve the understanding and diagnosis rate of this disease. Materials and methods The imaging data for seven adult patients pathologically diagnosed with adult PB were retrospectively analysed. Among the seven patients, six underwent computed tomography (CT) scans, two patients underwent abdominal magnetic resonance imaging (MRI), and five patients underwent 18F-FDG PET/CT. Results The tumours were located in the head of the pancreas in three cases, in the tail of the pancreas in two cases, and in the gastric antrum and neck of the pancreas in one case. Six tumours showed blurred edges, and an incomplete envelope was observed in only two cases when enhanced, which showed extruded growth and cyst-solid masses; one tumour was a solid mass with ossification. Showing mild or significant enhancement in the arterial phase (AP) for six cases. In the MRI sequence, isointensity was found on suppressed T1-weighted imaging, and hyperintensity was noted on suppressed T2-weighted imaging in two cases, with significant enhancement. Pancreatic duct dilatation was found in four cases. Tumour 18F-FDG PET/CT imaging exhibited high uptake in five cases. Conclusion Adult PB involves a single tumour and commonly manifests as cystic-solid masses with blurred edges. Capsules are rare, ossification is an important feature, tumours can also present in ectopic pancreatic tissues, with mild or strengthening in the AP, and 18F-FDG uptake is high. These features are relatively specific characteristics in adult PB.


Introduction
Pancreatoblastoma (PB) is a rare malignant embryonic tumour of the pancreas with multicellular differentiation [1][2][3]. Although PB can occur at any age, it occurs mostly in children and rarely in adults. Fewer than 50 cases have been reported since Palosaari et al. reported the first case of adult PB [4][5][6], and these studies describe only the clinical manifestations and pathological features of adult PB. Surgical resection is the mainstay of treatment, and complete resection has been associated with longterm survival [7]. As the neoplasm consists of multiple cell lines, squamoid corpuscles are distinctive pathological features of PB, and fine needle aspiration or cytology alone is not sufficient; therefore, the imaging techniques are particularly important and useful in diagnosing and staging adult PB.
The purpose of our study was to comprehensively describe the Imaging features of adult PB on computed tomography (CT), magnetic resonance imaging (MRI), and 18F-FDG PET/CT and to provide a literature review.

Clinical data
Imaging data for seven adult patients pathologically diagnosed with PB from November 2012 to April 2021 were retrospectively identified. We collected seven cases from Heyuan People's Hospital and Nanfang Hospital, Southern Medical University. The patients involved were aged from 26 to 69 years, with an average age of 56 years. This retrospective study was approved by the Ethics Committee of Heyuan People's Hospital and Nanfang Hospital, and informed consent was not required The clinical manifestations are shown in Table 1.

Imaging technique
Six patients underwent noncontrast and contrastenhanced CT, as shown in Table 2.
Two patients underwent abdominal MRI, as shown in Table 3.
Five examinations were carried out using a GE Discovery LS PET/CT scanner (GE Healthcare, Waukesha, Wisconsin, USA). 18 F-FDG with a radiochemical purity greater than 95% was obtained using a tracer synthesis system (TRACERlab FXFDG; GE Healthcare, USA). Approximately 60 min after intravenous injection of 318-524 MBq (8.6-14.2 mCi, 150 μCi/kg) of 18 F-FDG, wholebody PET/CT was performed using 120 kV, 80-250 mA, 0.5-s rotation time, 3.6 pitch, 5-mm slice thickness, and a 512 × 512 matrix without contrast enhancement from the mid-thigh to the head. The acquired CT and PET images were sent to an Xeleris workstation (GE Healthcare).   Lesions with increased uptake of 18F-FDG were considered positive tumours.

Image analysis
The CT and MRI images were evaluated by two radiologists (with 10 and 15 years of experience in abdominal radiology) who were blinded to patient information. Disagreements were resolved by consensus. The focuses of observation and analysis included tumour location, size, morphology, margin, capsule, growth pattern, internal components, contrast-enhanced scanning mode, the presence or absence of lymph node metastasis, adjacent blood vessels, and organ invasion and metastasis. 18 F-FDG PET/CT was used to analyse the uptake of the lesion (a lesion with uptake higher than that of the adjacent tissues was considered positive), and the brightness (indicating the uptake level) of the solid portion and necrotic area of the tumour was compared. The region of interest was drawn along the margin of the lesion on the PET image, and the standardized uptake value (SUVmax) was measured.

Pathology
All specimens in this study were reviewed by two pathologists with 10 years of experience to confirm the diagnosis of PB and the presence or absence of tumour necrosis, a capsule, lymph node metastasis, and vascular tumour thrombus.

Features of the imaging
The tumours were cyst-solid masses in five cases, and one was a solid mass, located in the head of the pancreas in three cases; two tumours were in the tail of the pancreas, one case was in the gastric antrum, and one case was in the neck of the pancreas (Fig. 1ac). Six tumours showed blurred edges, and an incomplete envelope noted in only two cases when enhanced (Fig. 2a). One case displayed ossification. Mild enhancement in the arterial phase (AP) was observed in two cases (Fig. 2b), and strengthened enhancement in the AP was observed in four cases (Fig. 2c). Pancreatic duct dilatation was found in four cases. In the MRI sequence, isointensity was found on suppressed T1-weighted (T1WI), and hyperintensity was found on T2-weighted suppressed (T2WI) in two cases ( Fig. 3ab), with significant enhancement (Fig. 3c). 18 F-FDG PET/CT had a high uptake (Fig. 4a-c) in five tumours.
Details are shown in Tables 4, 5, and 6.

Details of the pathology
In general, the tumour boundaries were well defined (Fig. 5a). Six tumours were greyish-white in the periphery and light yellow at the centre, and one tumour was light yellow. Microscopic examination revealed that all tumour tissues were acinar-like and duct-like with squamous corpuscles (Fig. 5b). Immature bone was visible in tumour cells in one case (Fig. 5c), and tumour cells were rich in interstitial fibroblasts in two cases. Immunohistochemical staining of acinar differentiation markers mainly included trypsin and chymotrypsin, and neuroendocrine markers mainly included CD56, CgA, Syn, and α-AT. The tumour was located in the gastric antrum, was quasi-round and cystic-solid, and showed restricted, bifurcated, and extruded growth on noncontrast CT scans

Discussion
Adult PB is a rare exocrine pancreatic malignancy, and the clinical manifestations are diverse and nonspecific. At present, the literature mainly focuses on case reports. Because of a lack of familiarity with this disease among clinicians and radiologists, the diagnosis is often inaccurate or delayed. Adult PB is usually misdiagnosed as other tumours by CT and/or MRI scans, However, the final diagnosis mainly depends on histopathological tests, but through multimodal imaging technology, with The gastric antrum tumours were mildly enhanced in the AP and not enhanced for necrosis and cysts in the centre on enhanced CT scans. c The tumour was significantly enhanced in the AP and not enhanced for necrosis and cysts in the centre on enhanced CT scans Fig. 3 a, b The tumour appeared isointense on T1WI and hyperintense on T2WI. c The tumour was indistinct with significant enhancement in the AP retrospective reanalysis and reevaluation of our study and in combination with supporting research, we found several imaging features indicating and/or supporting the diagnosis of adult PB. PB is a single mass, shown as a cyst-solid or solid mass, with cyst-solid being more common [8][9][10]. Corrias [5] found that most tumours are located in the head of the pancreas, and pancreas head tumours accounted for 43% of our cases. We support the head of the pancreas as the most common site. Ectopic pancreatic PB has not been reported in previous literature, and we found that it can occur in the gastric antrum in our study. These tumours were isodense on CT scans with low density in the centre and had sandy calcifications and ossification, which is consistent with PB consisting of multiple cell lines [11,12]. No pathology or imaging has been reported for ossification. We considered that pancreas tumours with ossification are evident on CT, and the possibility of adult PB should be considered, which is an important feature. PB was well circumscribed with a good capsule, and dilation of the pancreatic duct was rare [13,14]. In contrast to previous literature reports, we found that the edges of the masses were blurred, and an incomplete envelope was shown in only two cases when enhanced in our study, which is possibly related to PB having highly aggressive growth [15]. Pancreatic duct dilatation was found in our study, with tumour tissue in the pancreatic duct observed on microscopic examination, which indicates that pancreatic duct dilation caused by adult PB is common. Mild enhancement was observed in enhanced scans in a previous report [9]. Mild enhancement in the AP was observed in two tumours in our study, showing a slow-in and slowout pattern, similar to previous reports. In contrast to the literature reports, four cases had strengthened enhancement in the AP, showing a fast-in and slow-out pattern. The strengthened enhancement is related to the pathology finding of a vascular structure around the squamoid corpuscles, which is a new finding that will be helpful for diagnosing adult PB. In the MRI sequence, isointensity was found on T1WI, while hyperintensity was found on T2WI in two cases, with limited diffusion [9]. Contrastenhanced scans were consistent with CT scans, with significant enhancement in the AP [1,16]. MRI can more clearly show the malignant behaviour of adult PB and is helpful for clinicians to presurgically assess whether the tumour can be completely removed. With metastases at different sites, mainly including the liver followed by the lymph nodes [15,17], as in our study, a pancreatic tail lesion may invade the left kidney, which is in agreement with reports that PB is prone to invade adjacent structures.
The reviewed papers revealed [9] that the tumour does not exhibit FDG uptake on 18 F− FDG PET/CT examination. In our study, 18 F-FDG PET/CT had high uptake in five tumours. The SUV max average value was 10.56. One of the patients had pancreatitis, pancreas enlargement, an indistinct tumour on CT and MRI, and clear, high uptake on 18 F-FDG PET/CT. Zhou et al. [18] found that 18 F-FDG and 68 Ga-DOTATATE PET/MR were acquired for presurgical assessment of adult PB invasion and malignant potential, which revealed intense FDG uptake and mild DOTATATE uptake. We believe that 18 F-FDG PET/CT is an imaging modality for sensitive detection of pancreatic malignancies.
Adult PB and pancreatic cell tumours share many clinical and morphological similarities, and distinguishing between them is difficult. Thus, definitive diagnosis of PB depends on pathological examination characterized by the presence of squamous cell nests with prominent acinar differentiation and foci of ductal, squamous, and endocrine cells [19,20].
Limitations Because adult PB is extremely rare, this retrospective study had a small sample size. Our findings should be confirmed by further studies with large samples.
Follow-up Adults with PB have a worse prognosis than children with PB at 20 months following chemoradiotherapy and surgery [20]. The longest survival time among our patients was 26 months.
In conclusion, adult PB involves a single tumour and commonly manifests as cystic-solid masses with blurred edges. Capsules are rare, ossification is an important feature, tumours can also present in ectopic pancreatic tissues, with mild or strengthening in the AP, and 18 F-FDG uptake is high. Combining contrast CT and MRI, 18F-FDG PET/CT is the best choice for accurate diagnosis preoperatively, which is critical for directing future treatment.